I have an article on the Mills Horse Head Bonus machine that was originally printed in the Jan/Feb 1991 issue of Loose Change.
It was sent to me by a guy who bought the magazine on Flebay since all I wanted was the one article and didn't want to bid him up.
It has some good info on how the Bonus mech works.
If you have a Horse Head and would like a copy, send me a PM with your email address and I'll send it to you as a PDF file.